Method for creating a camera model for a camera of a surgical microscope, and arrangement having a surgical microscope

ABSTRACT

A method for creating a camera model for a camera of a surgical microscope includes positioning a calibration object in an initial pose in an observation region of the camera, determining a pose delta for reaching a first pose for the calibration object in a measurement space of the camera starting from the initial pose, positioning the calibration object in the first pose in accordance with the determined pose delta, making a recording of the calibration object in the first pose with the camera, positioning the calibration object in at least one further pose, making a recording of the calibration object in the at least one further pose, and creating a camera model based on the recordings made, the first pose and the at least one further pose being chosen with a distribution in the measurement space such that a camera model is obtained which represents the entire measurement space.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority to German patent application DE 10 2021112 737.8, filed May 17, 2021, the entire content of which isincorporated herein by reference.

TECHNICAL FIELD

The disclosure relates to a method for creating a camera model for acamera of a surgical microscope, to a method for estimating a pose of anobject in a measurement space of a camera of a surgical microscope, to amethod for verifying a camera model, to an arrangement having a surgicalmicroscope, to a computer program and to a computer-readable medium.

BACKGROUND

Surgical microscopes are used in various medical disciplines, such asophthalmic surgery, dental surgery or neurosurgery. On account of theincreasing complexity of surgical microscopes, there is an increasedneed for testing, adjustment and calibration methods to test thefunctionality and to be able to undertake changes or apply specificsettings where necessary. These methods, which are subsumed by the termcalibration method below, can be carried out during the intended use ofthe surgical microscope, or else within the scope of manufacture, andservicing and upkeep.

Calibration methods within the aforementioned meaning may include, interalia: diopter settings on the eyepiece, verifying and setting the X, Y,Z-position of cameras and other optical components, verifying andsetting intrinsic camera parameters, temporal calibration of cameras,verifying and setting zoom and autofocus, establishing the absolutefluorescence intensity, verifying and setting the kinematics of thestand for improving the absolute positioning accuracy.

By way of example, the accuracy of pose estimates is decisivelyinfluenced by the intrinsic calibration of the camera. Importantparameters, for example the position of the camera chip relative to theoptical axis of the camera system and distortion coefficients, aredetermined within the scope of the intrinsic calibration on the basis ofrecordings of a calibration object, which is also referred to ascalibration standard, calibration target or marker. The quality of theparameter determination, i.e., the measure for the deviation between thedetermined parameter and the actual parameter, depends, inter alia, onthe pose from which the recordings are made.

To verify and set the surround camera for tracking instruments, the(hand-held) surgical instrument to be tracked is provided with atwo-dimensional (2D) calibration object. The calibration object isfilmed by a camera attached to the microscope and its relative positionin the operating field is calculated. With appropriate driving of themotor-driven stand of the microscope, it is possible for the opticalaxis of the microscope to track the tip of the surgical instrument.

The intrinsic calibration of the surround camera is required tofacilitate this. The intrinsic parameters serve to re-establish therelationship between the camera coordinate system and image coordinatesystem. Moreover, the distortion coefficients of the optical unit can bedetermined.

A further precondition for tracking instruments is the calibrationbetween the coordinate origin of the surround camera and a physicalpoint on the microscope since the movement of the instrument, asrecognized by the surround camera, must be implemented in the coordinatesystem of the microscope. On account of the coordinate origin of thesurround camera corresponding to no physical point on the surroundcamera, this cannot be measured geometrically. Therefore, methods suchas the hand/eye calibration are required to carry out this calibration.

To check and set internal cameras, for example for the topographicreconstruction of the surface of the site with at least two cameras of astereoscopic microscope or for expanded representations (augmentation),an intrinsic calibration (see the explanation above) and an extrinsiccalibration are required. In the case of the extrinsic calibration, thespatial arrangement of the cameras, i.e., the rotation and translationthereof, with respect to one another is determined. Since the cameras ina surgical microscope are arranged in the optical path downstream of amovable lens system for zoom and focus, there is a particular challengein carrying out the calibration for arbitrary zoom and focus values, andin compensating tolerances in the motors which move the lenses for zoomand focus. An accurate calibration is also required if image data of acamera should be superposed on image data of another camera or be usedfor augmentation purposes thereon.

The quality of the results of some applications depends on the absolutepositioning accuracy of the stand of the microscope. Examples of thisinclude the pivoting of the microscope about the focal point or theoff-line positioning in relation to certain poses in space, e.g., withinthe scope of tool tracking. The absolute positioning accuracy of thestand depends, in turn, on the exact knowledge of the kinematicparameters of the stand, which may deviate from the nominal values onaccount of usual assembly and manufacturing tolerances.

By way of example, the stand kinematics can be calibrated by virtue ofone or more defined points in space being approached in differentorientations of the stand. In the specific example of a surgicalmicroscope, this can be realized by virtue of the optical unit of thesurgical microscope being aligned on a calibration object. Together withthe camera calibration, it is then possible to determine the actual poseand orientation of the surgical microscope relative to the calibrationobject. In knowledge of the fact that the calibration object is arrangedat the same locations in all poses, the deviation of the actual posefrom the nominal pose of the surgical microscope represents the variableof the calibration algorithm to be minimized. To be able to calibrate alarger working region, it is also possible to use a plurality ofcalibration objects with the fixed and known pose in relation to oneanother.

The calibration methods known from the prior art are based on randomrecordings of the calibration object, using which more or lesssuccessful attempts are made to map the measurement space of the camera.To this end, a calibration object is arranged in the volume observableby the camera, independently of the size and position of the measurementspace. This leads to a camera model based on such recordings notreproducibly mapping the entire measurement space. Inaccuracies arise inthe underrepresented regions of the measurement space. Moreover,currently available methods do not allow an automated calibration of asurgical microscope in the region of use.

Making recordings from different observation angles and/or differentdistances for calibration purposes has only been disclosed in thesubsequently published German patent application with the file reference10 2019 131 646.4, filing date: 22 Nov. 2019, by the applicant of thepresent application. This procedure can be implemented automatically,for example by virtue of a robotic stand being used to align the opticalobservation unit.

SUMMARY

Against this background, it is an object of the present disclosure toprovide a method that can be used to create a camera model for a cameraof a surgical microscope, the model having a high accuracy over theentire measurement space. Further objects of the present disclosure areproviding a method for estimating a pose of an object in a measurementspace of a camera of a surgical microscope, providing a method forverifying a camera model, an arrangement, and providing a computerprogram.

The objects are achieved by a method for creating a camera model for acamera of a surgical microscope, a method for estimating a pose of anobject, a method for verifying a camera model, an augmentation, anarrangement, and a non-transitory computer-readable storage medium asdescribed herein.

A first aspect of the disclosure relates to a method for creating acamera model for a camera of a surgical microscope. The method includesthe following method steps: positioning a calibration object in aninitial pose in an observation region of a camera of the surgicalmicroscope, determining a pose delta for reaching a first pose for thecalibration object in a measurement space of the camera starting fromthe initial pose, positioning the calibration object in the first posein accordance with the determined pose delta, making a recording of thecalibration object in the first pose with the camera, positioning thecalibration object in at least one further pose in the measurement spaceof the camera, making a recording of the calibration object in the atleast one further pose with the camera, and creating a camera model onthe basis of the recordings made. In this case, the first pose and theat least one further pose are chosen with such a distribution in themeasurement space that a camera model is obtained which isrepresentative in relation to the entire measurement space.

The method can be carried out in computer-implemented fashion, that isto say at least one method step can be carried out by a computerprogram.

A camera model is representative in relation to the entire measurementspace if the pose of an object whose geometry and actual pose are knowncan be estimated in such a way with the aid of this model that the errorin the pose estimate, that is to say the deviation between the estimatedpose and the actual pose, is below a target limit in the demandedmeasurement space, that is to say a specifiable quality of the cameramodel is achieved. Should the pose estimate have an error, the latter istypically of the same magnitude in the entire measurement space.

In an exemplary embodiment, the surgical microscope includes one or morecameras with associated stands. The cameras can be used to generatephotographs or films of an observation object or observation region, forexample the site. To this end, the camera includes an image sensor orimage chip and a lens. The camera can be a surround camera, for examplea surround camera with a fixed focal length and without a zoom system,or a microscope camera.

A surround camera can be understood to mean a camera whose measurementspace images the surround of the operating site, and which can be usedfor tool tracking, for example. A microscope camera can be understood tomean a camera which images the immediate operating site, in particularimaging the latter in a magnified fashion. The microscope camera mayalso be referred to as principal observer. The measurement space of themicroscope camera usually has a smaller volume than the measurementspace of the surround camera. By way of example, the diameter of thecylindrical measurement space of the microscope camera may be smallerthan the diameter of the cylindrical measurement space of the surroundcamera.

Consequently, the method can be used to create a camera model of asurround camera or a microscope camera. Optionally, the method can beused to create a camera model both for the surround camera and for themicroscope camera. For the latter variant, a camera model for thesurround camera can initially be created in accordance with the methodand a camera model for the microscope camera can subsequently be createdin accordance with the method, the sequence also being able to be chosenin reverse. Moreover, there is the option of initially creating a cameramodel according to the method for one of the two cameras, that is to saythe surround camera or the microscope camera, and of creating a cameramodel for the respective other camera of the two cameras on the basis ofthis camera model created first, by virtue of implementing geometrictransformation of the coordinate system of the one camera into thecoordinate system of the other camera.

A stand serves to position, align and hold a camera. To this end, thecamera can be connected to the stand with a mount. To fulfil this task,the camera held by the stand should firstly be able to be positionedwith as little resistance as possible. Secondly, the camera, oncepositioned, should be able to be held securely in its position.Additionally, the stand can fulfil other tasks, for example offacilitating a tracking of instruments by virtue of the camera beingmoved in a targeted fashion with the stand.

In order to be able to fulfil these tasks, the stand includes aplurality of stand links, interconnected in secured or articulatedfashion, for example a height-adjustable stand column, a support arm, aspring arm, and a mount for the optical observation unit. Moreover,provision can be made of a stand base, on the underside of whichdevices, e.g., rollers, for displacing the stand may be attached. Thespecific embodiment of the stand depends, inter alia, on the dimensionsof the camera, the desired application, e.g., during an operation, andthe space available at the setup location.

The stand can have a motor-driven configuration such that positioningand alignment of the camera can be facilitated by an appropriate controlof the motors of the stand. For this purpose, the stand can besignal-connected to a control unit.

By moving the stand, it is likewise possible to move the camera, and sodifferent observation positions can be adopted.

Optionally, the surgical microscope may include a control unit that isconfigured and designed to output control signals to the stand and/orthe camera, for example to carry out the method for creating a cameramodel or the method, described below, for calibrating a surgicalmicroscope.

To this end, there can be a signal-transmitting operative connectionbetween the control unit and motors of the stand and/or adjustmentdevices of the camera such that it is possible to output control signalswhich bring about certain positioning of the stand and the camera.

By way of example, the output of the control signals can be implementedas a consequence of an input using an input unit connected to thecontrol unit, for example should a user manually initiate one of theaforementioned methods by way of an appropriate input. Alternatively,the control signals can be output in response to the presence of othertriggers, e.g., expiry of a specifiable time interval, expiry of aspecifiable use duration of the surgical microscope, a change in thelocation of the surgical microscope, an excessive deviation of an actualvalue from a target value, etc.

The control unit facilitates a partly or fully automated implementationof the specified methods, and so the said methods can be carried outwith little outlay in terms of time and staff, e.g., even without thepresence of the user. Thus, calibration methods can also be carried outoutside of the period of use of the surgical microscope, e.g., at nightor over the weekend. Moreover, the number of error sources is reduced asa result of the automation, since a user intervention is not required oronly required to a small extent. The reliability and reproducibility ofthe measurement results obtainable by the surgical microscope can beincreased as a result.

In conjunction with the present disclosure, a calibration object has atleast one characteristic marking, typically a plurality ofcharacteristic markings, for example corners of a chequerboard pattern,the properties of which, e.g., size, distance, and alignment, are knownaccurately. By way of example, the calibration object can be in the formof a two-dimensional calibration pattern or three-dimensionalcalibration body.

By way of example, a two-dimensional calibration pattern can be embodiedas a chequerboard pattern, a point pattern, a QR code, a logo, or thelike. It is easily and cost-effectively possible to produce such acalibration pattern and to arrange the latter optionally also on a standof the surgical microscope, e.g., with printing, laser engraving oradhesive bonding. Furthermore, there is the option of cost-effectivelyrepresenting a two-dimensional calibration pattern on a monitor, withthis rendering a simple change between different calibration patternspossible. Moreover, already available monitors can be used for therepresentation.

A three-dimensional calibration body can include a main body that istransparent in the spectral range employed, i.e., for example, in thevisible spectral range, and one or more non-transparent calibrationmarks arranged in the main body. Such a calibration body facilitates acalibration in three dimensions. In respect of further details ofpossible three-dimensional calibration bodies, reference is made to theGerman patent application DE 10 2018 115 824 A1, which describes suchcalibration bodies in detail.

The calibration object can be configured such that the characteristicmarkings are visible in the visible spectral range, e.g., in aconventional white-light image. In addition or as an alternative to thevisible spectral range, the characteristic markings may be visible in adifferent spectral range, e.g., in the infrared spectral range.

The calibration object can have a passive, i.e., always present, form,e.g., as a printed pattern, or it can have an activatable anddeactivatable form. Activatable and deactivatable means that thecalibration object or the structures, marks, etc., used for thecalibration can be activated or deactivated according to need, e.g., bytargeted driving of light-emitting diodes or in the form of acalibration pattern that is dynamically displayed on a monitor.

In conjunction with the present disclosure, the term “pose” isunderstood to mean the combination of position and orientation of thespecified objects or components or reference axes in three-dimensionalspace (see also DIN EN ISO 8373). The position of a punctiform mass inrelation to a Cartesian coordinate system is accordingly defined by thedistances in the coordinate directions x, y, z. If a second Cartesiancoordinate system is spanned at this point of mass, the orientation ofthis coordinate plane is defined by the angular offset of the latter'scoordinate axes in relation to the corresponding axes of the basecoordinate system. Three angles are additionally required, thesedescribing the relative position of the new coordinate system inrelation to the base coordinate system.

In the present case, the term “measurement space” is understood to meanthe volume to be observed with the camera, that is to say the region inwhich the processes to be observed take place and for which acalibration must be available, for example in order to be able touniquely determine distances. The maximum region able to be imaged withthe camera may be larger by comparison, that is to say the measurementspace is restricted to a part of the volume even observable with thecamera. In the present case, the volume observable with the camera isreferred to as “observation region”.

The measurement space may be defined in the coordinate system of thecamera for which a camera model is created in accordance with theproposed method. Alternatively, the measurement space may be defined inthe coordinate system of a further camera of the surgical microscope.For the latter alternative, the positioning of this further camera inrelation to the camera for which the camera model is created must beknown, that is to say the geometric transformation from the coordinatesystem of the further camera, e.g., the microscope camera, to thecoordinate system of the camera for which the camera model is created,e.g., the surround camera, must be known.

By way of example, the measurement space may have a cylindrical form.Depending on the distance from the camera, the measurement space can besubdivided into a near region and a remote region. By way of example,the near region may be localized between 0% and 30% of the longitudinalextent of the measurement space along the optical axis of the camera andthe remote region, by contrast, may be localized between 70% and 100%.

In the first method step of the provided method, a calibration object ispositioned in an initial pose in the observation space of the camera,that is to say the calibration object positioned in the initial pose mayalso be located outside of the measurement space for as long as it isdetectable by the camera. The initial pose can be chosen as desiredwithin the observation region. The initial pose can be estimated with aninitial camera model or a nominal camera model.

In the next step, the pose delta between the initial pose and a firstpose is determined, that is to say the pose delta that is required toreach the first pose starting from the initial pose. In this case, thepose delta corresponds to the vector according to which the calibrationobject needs to be moved in order to convert the initial pose into thefirst pose. Optionally, a transformation of the coordinate systems toone another needs to be taken into account, depending on the cameracoordinate system in which the measurement space is defined.Subsequently, the calibration object is positioned in the first pose inaccordance with the determined pose delta. To this end, the pose deltacan be controlled with the stand of the surgical microscope, forexample, in order to change the camera pose and hence indirectly changethe pose of the calibration object. In contrast to the initial pose, thecalibration object positioned in the first pose is necessarily at leastpartially within the measurement space.

In a further step, a recording, for example a photographic or videorecording, of the calibration object is made. Optionally, thecalibration object can be activated before the recordings are made, forexample by virtue of the calibration object being illuminated or aself-luminous calibration object being activated to shine.

Subsequently, these two steps are repeated for at least one furtherpose. In this case, the overall number of poses depends, inter alia, onthe number of characteristic markings on the calibration object (themore markings, the fewer poses are required), the size of themeasurement space (the larger the measurement space, the more poses arerequired) and the required accuracy or quality of the camera model (thehigher the accuracy, the more poses are required). By way of example,the overall number of poses can range between 20 and 25.

The recordings made are subsequently evaluated and a camera model iscreated on the basis of the totality of the recordings made. The cameramodel describes, inter alia, the properties of the image sensor of thecamera and of the camera optics, and the arrangement of image sensor andoptics in relation to one another. To this end, the camera modelincludes values for calibration parameters, that is to say correctionfactors, with which it is possible to take account of the relationshipbetween a beam in the measurement space of the surgical microscope, moreprecisely in the measurement space of the camera of the surgicalmicroscope, and a point on the image sensor of the camera. A pinholecamera model may form the basis to this end, that is to say a deviationfrom a nominal pinhole camera model can be compensated with thecalibration parameters obtained in the camera model. Expresseddifferently, the camera model may include values for calibrationparameters, that is to say correction factors, with which it is possibleto take account of the relationship between the position of the opticalcenter according to the pinhole camera model and a point on the imagesensor of the camera. Moreover, the camera model may include correctionfactors that can be used to correct distortions, so-called distortioncoefficients.

By way of example, the camera model may include values for the followingcalibration parameters: distance between image sensor of the camera andoptical center in the x-direction, y-direction, and z-direction, anddistortion coefficients.

Optionally, provision can be made for the value of a quality parameterto be determined for the camera model created from the totality of therecordings made. By way of example, this value of the quality parametercan be used to ensure a certain tracking or data overlay quality. Tothis end, the established value of the quality parameter can be comparedto a specifiable target value or limit value.

To create the camera model, the characteristic markings of thecalibration object can be recognized with the aid of suitable algorithmsin a recording of the calibration object and can be assigned to oneanother. Thus, the points in the image of the recording corresponding tothe characteristic markings and the geometry of the calibration objectare known. The calibration parameters can be determined therefrom withan optimization algorithm, so that the camera model maps thetransformation of the calibration object on the image sensor of thecamera to the best possible extent. In this case, the optimizationalgorithm can take account of not only one recording, but of a pluralityor all recordings of the calibration object in the various poses.

The made recordings can be evaluated in computer-implemented orsoftware-based fashion, either in the surgical microscope itself or withan external evaluation unit, which is signal-connected to the surgicalmicroscope.

Provision is made for the first pose and the at least one further poseto be chosen with such a distribution in the measurement space, i.e.,defined in a targeted manner, that a camera model is obtained which isrepresentative in relation to the entire measurement space.

Expressed differently, it is not only a certain number of poses of thecalibration object that are defined, and corresponding recordings aremade, but the poses to be adopted are determined in a targeted fashionto the effect of the measurement space being sufficiently accuratelyrepresented at all points, i.e., that no region of the measurement spaceis overweighted or underweighted. Sufficiently accurately means that theaccuracy for a pose estimate for which the camera model is used is abovea certain limit, i.e., the error of the pose estimate in the requiredmeasurement space is below a target limit. Symmetries of the measurementspace can be taken into account in the process.

As a result, a camera model with a high accuracy or calibration qualityis advantageously created since the values for the calibrationparameters can be estimated better, i.e., more accurately, and with agreater reproducibility for the entire measurement space. This directlyhas an effect on the quality of the pose estimates and measurementscarried out using such a camera model, said pose estimates andmeasurements likewise being more accurate and having a greaterreproducibility accordingly.

The targeted definition of the poses moreover facilitates an automationof the method since there is no need for a manual movement of thecalibration object and/or of the surgical microscope. This may alsocontribute to an increase in the reproducibility over random recordings.Moreover, the method can even be started by non-specialist staff, forexample a theatre nurse, e.g., by operating a simple start button. Themethod can likewise be carried out within the scope of remotemaintenance.

Typically, the camera model can be created with a single calibrationobject as the latter is usable multiple times in various poses.

Moreover, the calibration object may have a transportable design, thatis to say it is not connected to the surgical microscope, e.g., thestand thereof. Connectability of the calibration object to the surgicalmicroscope or a body arranged in stationary fashion in relation to thesurgical microscope, for example a wall, etc., for the purposes offacilitating a certain pose in relation to the surgical microscope isnot required either since the pose of the calibration object need not beknown. Expressed differently, the calibration object can be positionedfreely in space, i.e., with an unknown pose, for example can be placedon a tabletop.

The creation of the camera model when freely positioning the calibrationobject in space can be achieved by virtue of the initial pose(calibration object to camera), which is initially unknown on account ofthe free positioning, being estimated or calculated. Expresseddifferently, when positioning the calibration object freely in space,the method step of positioning the calibration object in an initial posein an observation region of the camera of the surgical microscopeincludes an estimation, e.g., with an initial camera model or nominalcamera model, or calculation of the initial pose.

Subsequently, the pose delta for reaching the first pose can bedetermined and, as already explained, the calibration object can bepositioned in the first pose in accordance with the determined posedelta, for example with an appropriate displacement of the camera by wayof the robotic stand system.

By contrast, a fixed trajectory is always traversed in calibrationmethods known from the prior art. This requires the geometrictransformation of the pose of the calibration object in relation to areference point, e.g., the foot of the stand, and hence the initial poseto already been known at the outset, i.e., free positioning thecalibration object in space is precisely not possible.

The same calibration object can also be used to create the camera modelsfor a plurality of surgical microscopes or other optical observationdevices. By way of example, it is therefore sufficient for a technicianto carry along a single calibration object which can be used to createthe camera models of different surgical microscopes or opticalobservation devices, for example at different locations. As a result,the costs of further calibration objects can be saved and userestrictions, as a result of calibration objects being arranged on theoptical device, can be avoided.

By way of example, creation of the camera model can be carried out firstwhen putting the surgical microscope into operation and can besubsequently carried out at certain time intervals or depending oncertain events, for example transportation, temperature variations, inorder to always be able to obtain accurate and reliable measurementresults or pose estimates with the camera.

The created camera model can be stored in a memory unit. By way ofexample, the camera model can be stored on a computer-readable medium.

Optionally, the method can provide for one or more verification steps,in which a check is carried out as to whether the first or a furtherpose was in fact reached. In the case of a deviation there can be acorresponding correction or repetition. The method overall can also becarried out multiple times in order to achieve a greater accuracy. Inthis case, the camera model respectively created previously can be usedas initial camera model.

According to various embodiment variants, a first number of poses can belocated in the region near to the camera and a second number of posescan be located in the region remote from the camera. Typically, thenumber of poses in the near region may correspond to the number of posesin the remote region in this case.

To obtain the best possible calibration results, recordings should berecorded with the most homogeneous distribution possible over the entiremeasurement space, ideally having an image in each measurement point.However, this would cause much outlay in terms of time and computation.In order to reduce the outlay but nevertheless map the measurement spacerepresentatively, it is possible to record recordings, e.g., only in oneplane, typically at a mid-distance. A disadvantage in this case wouldlie in an underrepresentation of the distortion, which predominantlyoccurs in the near region (a small distance between camera andcalibration object causes distortions, especially in the edge region).To reduce this effect, recordings can be recorded in two or more planes,for example in the near region and remote region.

By using poses in the near region and remote region, a high accuracy ofthe camera model is also achieved in these regions. The number of posesin the near region and remote region being the same ensures that noregion is overweighted. Provided the required accuracy of the cameramodel or calibration quality has already been achieved, it is possibleto dispense with poses between the near region and remote region. Thiscan reduce the overall number of poses, and so the camera model can becreated quicker and with less computational outlay.

According to further embodiment variants, the poses can be chosen insuch a way that the calibration object is positioned fully within themeasurement space in each pose. Expressed differently, allcharacteristic markings to be evaluated can typically be situated withinthe measurement space.

As a result, the calibration object will advantageously only be imagedon the image sensor of the camera in the required measurement space.Characteristic markings of the calibration object located outside of themeasurement space consequently remain unconsidered when creating thecamera model, preventing a falsification of the camera model and beingable to contribute to higher accuracy of the camera model.

Should characteristic markings of the calibration object nevertheless belocated outside of the measurement space and be recorded by the imagesensor of the camera, these characteristic markings can be eliminatedwithin the scope of a post-calculation and can remain unconsidered whencreating the camera model and during subsequent calibrations.

According to further embodiment variants, the calibration object can bepositioned in the measurement space by changing the position of thecamera.

Expressed differently, there is the option of not moving the calibrationobject itself for the purposes of changing the pose vis-à-vis the cameraor the measurement space, but of appropriately displacing themeasurement space by moving the camera.

As a result, the provided method can be simplified since it issufficient to position the calibration object once, for example place iton a tabletop. Then, the poses can be adopted by varying the cameraposition with the stand. This also corresponds to the usual way ofhandling the surgical microscope since, for example during an operation,it is not the position of the site that is changed but the cameraposition. Moreover, the use of a robotic stand renders an automation ofthe method possible, which is connected to the advantages alreadydescribed above.

A further aspect of the disclosure relates to a method for estimating apose of an object in a measurement space of a camera of a surgicalmicroscope. In this case, the pose of the object, for example of asurgical instrument, is estimated, for example within the scope of whatis known as tool tracking, using a camera model for the camera of thesurgical microscope which was created with a method according to thedescription above. Consequently, the method for estimating the pose isaccordingly linked with the advantages of the camera model and themethod for creating a camera model, and so reference can be made to theexplanations in this context.

The use of such a camera model advantageously facilitates a poseestimate with a high accuracy. In addition, the pose estimate can bewholly or partly carried out in computer-implemented fashion and, as aconsequence, in wholly or partly automated fashion. To this end, thecamera model can be retrieved from a memory unit in which it is stored.Such a memory unit may be integrated in the surgical microscope or maybe signal-connected to the surgical microscope.

Alternatively or in addition, the camera model created with one of theabove-described methods can be used for data overlay. To this end,preoperative data can be overlaid in a camera recording using thecreated representative camera model. The use of a camera model createdwith one of the above-described methods advantageously facilitateshigh-accuracy data overlay.

A further aspect of the disclosure relates to a method for verifying acamera model which was created with a method according to the precedingdescription.

The verification method includes: determining a current pose of acalibration object positioned in a measurement space of a camera of thesurgical microscope, defining a target pose for the calibration object,determining a pose delta for reaching the target pose starting from thecurrent pose, changing the current pose of the calibration object inaccordance with the determined pose delta, determining a deviationbetween the set target pose and the actual target pose, and comparingthe determined deviation with a limit for a maximum admissibledeviation.

The change in the position of the calibration object in accordance withthe determined pose delta can be implemented manually, for example byvirtue of the determined pose delta being displayed on a monitor and thecamera being displaced with the stand following a manual input by theuser, for example a servicing technician, hospital staff member,assembly staff member in the manufacturing line. Alternatively, thechange in position can be implemented in automated fashion by virtue ofthe pose delta being implemented by the robotic stand system.

Depending on the application, the limit may be defined on an individualbasis and may have an absolute or relative value.

Optionally, the method steps can be repeated in order to be able todetermine the deviation for different target poses, for example in thenear region and remote region. This may facilitate a more accurateverification of the camera model.

The verification method facilitates a simple and quick verification ofthe camera model. By way of example, it can be carried out at certaintime intervals or on the basis of certain events, for example transport,temperature variation. Should the determined deviation be above thelimit, a new camera model can be created forthwith, and so outage timescan be reduced, and reliable and reproducible measurement results canmoreover be obtained at all times.

In this case, there is the option of reducing the overall number ofposes when creating a new camera model, and so the time required forcreating the camera model can be reduced.

A further aspect of the disclosure relates to an arrangement comprisinga surgical microscope with a camera and means suitable for carrying outone of the methods explained above, i.e., a method for creating a cameramodel, a method for calibrating the surgical microscope, and/or a methodfor verifying a calibration.

Consequently, the advantages of the methods are correspondinglyconnected with the arrangement. Reference is made to the explanationsgiven above.

By way of example, the camera can be a surround camera or a microscopecamera. In an exemplary embodiment, the surgical microscope may includeboth a microscope camera and a surround camera.

Optionally, the surgical microscope may have a stand, likewise asdescribed above, so that automation is rendered possible. The means forcarrying out the methods may include a control unit configured anddesigned to output control signals to the stand and/or the camera forthe purposes of carrying out one of the aforementioned methods, forexample for positioning the calibration object and/or changing thecurrent pose of the calibration object in accordance with a determinedpose delta, a memory unit for storing the camera model and optionallystoring the recordings, and a processing unit for processing therecordings made, that is to say for creating the camera model on thebasis of the recordings. Optionally, the processing unit can beconfigured to estimate a pose of an object in the measurement space withthe camera model. Further optionally, the processing unit can bedesigned to define a target pose for the calibration object, determine apose delta for reaching the target pose starting from the current pose,determine a deviation between the set target pose and the actual targetpose, and compare the determined deviation with a limit for a maximumadmissible deviation such that a verification of the camera model isrendered possible.

If provision is made for the camera model to be created starting fromfreely positioning the calibration object in space, the arrangement mayinclude means for estimating or calculating the initial pose. By way ofexample, the arrangement may include a processing unit which isconfigured and designed to estimate the initial pose. To this end, theprocessing unit can use an initial camera model or nominal camera modelwhich is stored in the memory unit and retrieved therefrom, whereforethe processing unit and the memory unit can be operativelysignal-connected to one another.

A further aspect of the disclosure relates to a computer programincluding commands which cause an arrangement according to thedescription above to carry out one of the methods as explained above,that is to say a method for creating a camera model, a method forestimating a pose of an object in a measurement space of a camera of asurgical microscope, and/or a method for verifying a camera model.

A computer program can be understood to mean a program code that isstorable on a suitable medium and/or retrievable from a suitable medium.Consequently, the computer program can be stored on a computer-readablemedium, e.g., a computer-readable data medium. Furthermore, a datacarrier signal can be formed, which transmits the computer program.

Any medium suitable for storing software, for example a non-volatilenon-transitory memory installed in a controller, a DVD, a USB stick, aflash card or the like, can be used to store the program code. By way ofexample, the program code can be called via the Internet or an intranetor via another suitable wireless or wired network.

BRIEF DESCRIPTION OF THE DRAWINGS

The disclosure will now be described with reference to the drawingswherein:

FIG. 1 shows a surgical microscope having a stand and a camera accordingto an exemplary embodiment of the disclosure.

FIG. 2 shows the degrees of freedom provided by the stand of FIG. 1 andits mount.

FIG. 3 shows a flowchart of a method for creating a camera model andestimating a pose according to an exemplary embodiment of thedisclosure.

FIG. 4 shows a schematic representation of a surgical microscope and acalibration object in an initial pose.

FIG. 5 shows a schematic representation of a surgical microscope and acalibration object while creating a camera model.

FIG. 6 shows a schematic representation of the surgical microscopeduring the creation of the camera model, with various poses of thecalibration object.

FIGS. 7A to 7D show various poses of the calibration object in the nearregion.

FIGS. 8A to 8D show various poses of the calibration object in theremote region.

FIG. 9 shows a flowchart of an exemplary method for verifying a cameramodel.

FIG. 10 shows a schematic representation of a surgical microscope and acalibration object in a current pose.

FIG. 11 shows a schematic representation of the surgical microscope andthe calibration object in a target pose.

FIG. 12 shows a schematic representation of an arrangement according toan exemplary embodiment of the disclosure.

FIG. 13 shows a schematic representation in relation to an alternativedefinition of the measurement space.

DESCRIPTION OF EXEMPLARY EMBODIMENTS

FIG. 1 depicts a surgical microscope 100 including a motor-driven stand201 and an optical observation unit 102 fastened to the stand 201 andincluding a camera 103 and an eyepiece 104. The camera 103 is themicroscope camera of the surgical microscope 100, i.e., the mainobserver. Alternatively, the camera 103 can be a surround camera.Optionally, the surgical microscope 100 can include both a camera 103,e.g., a microscope camera, and a further camera 108 (see FIG. 13), e.g.,a surround camera. By entering navigation data, the optical observationunit 102 can be automatically set in terms of its orientation andposition, which also allows remote positioning and orientation of theoptical observation unit 102 in such a way that a certain section of anobject field, e.g., the site, is displayed in optimal fashion. For thispurpose, a controller or control unit 401 is assigned to the stand 201,said control unit undertaking the positioning and orientation of theoptical observation unit 102 on the basis of received position and/ororientation control data by virtue of control signals 403, 404 beingoutput to suitable actuators.

Below, the stand 201 and the degrees of freedom facilitated by the standfor the optical observation unit 102 are described in more detail on thebasis of FIGS. 1 and 2. In the example of a stand 201 shown in FIG. 1,the stand 201 rests on a stand base 205 which has rollers 206 on thelower side thereof, said rollers enabling a displacement of the stand201. In order to prevent an unwanted displacement of the stand 201, thestand base 205 includes a foot brake 207.

As stand links, the actual stand 201 includes a height-adjustable standcolumn 208, a support arm 209, a spring arm 210, and a mount 211 for theoptical observation unit 102, which in turn includes a connectionelement 213, a swivel arm 215 and a holding arm 214. The degrees offreedom provided by the stand links for positioning the opticalobservation unit 102 are shown in FIG. 2. At its one end, the supportarm 209 is connected to the stand column 208 in a manner rotatable aboutan axis A. At the other end of the support arm 209, one end of thespring arm 210 is fastened in a manner rotatable about an axis B that isparallel to the axis A such that the support arm 209 and the spring arm210 form an articulated arm. The other end of the spring arm 210 isformed by a tilt mechanism (not depicted here), on which the mount 211is fastened and which enables a tilting of the mount 211 about the axisC.

The mount 211 has an axis of rotation D, a swivel axis E, and a tiltaxis F, about which the optical observation unit 102 can be rotated,swiveled, and tilted, respectively. Using a connection element 213, themount 211 is fastened at the outer end of the spring arm 210 in a mannerrotatable about the axis of rotation D. The axis of rotation D extendsalong the connection element 213. The connection element 213 is adjoinedby a swivel arm 215, with the aid of which the optical observation unit102, more precisely a holding arm 214 which is attached to the swivelarm 215 and on which holding arm the optical observation unit 102 isfastened with a holder (not illustrated), can be swiveled about theswivel axis E. The swivel axis E extends through the swivel arm 215. Theangle between the swivel arm 215 and the connection element 213, i.e.,the angle between the swivel axis E and the axis of rotation D, can bevaried with an adjustment mechanism arranged between the connection part213 and the swivel arm 215.

The tilt axis F, which enables tilting of the optical observation unit102, extends through the holding arm 214 in a manner perpendicular tothe plane of the illustration. The optical observation unit 102 isfastened to the holding arm 214 with a holder (not illustrated here).

The degrees of freedom of the mount 211 and the adjustment options ofthe optical observation unit 102, e.g., focusing, sharpness,magnification factor, etc., can be set with an actuating device 202,which is illustrated as a foot control panel in the present exemplaryembodiment. However, the actuating device 202 can also be realized as ahand-operated switching element or as a combination of foot- andhand-operated switching element. Moreover, a remote control can befacilitated.

Even if the stand 201 has been described on the basis of a specificexample, a person skilled in the art will recognize that differentlyformed stands can also find use.

The camera 103 of the surgical microscope 100 described in exemplaryfashion with reference to FIGS. 1 and 2 must be intrinsically calibratedfor various measuring methods, i.e., a camera model needs to be created.Optionally, the camera model to be created can be used for furthercalibrations, e.g., a hand/eye calibration of the camera 103 or acalibration of the kinematic mechanism of the stand 201.

An exemplary method for creating a camera model and a method forestimating a pose of an object are explained below with reference toFIGS. 3 to 8. FIG. 3 shows a flowchart in this respect, with the cameramodel being created in method steps S1 to S7 and the created cameramodel being used in method step S8 for the purposes of estimating thepose.

A two-dimensional calibration pattern in the form of a chequerboardpattern serves as a calibration object 300 in the exemplary embodiment.Characteristic markings that are usable for creating the camera modelare for example those points of the chequerboard pattern where black andwhite fields are adjacent to one another. Moreover, the dimensions andangles of the black or white fields can also be used for creating thecamera model.

In the exemplary embodiment, the measurement space 301 of the camera 103has a circular cylindrical form, the longitudinal axis of the cylindercorresponding to the optical axis of the camera 103. Consequently, themeasurement space 301 is rotationally symmetric in relation to theoptical axis OA. The measurement space 301 has a near region 105 and aremote region 106 (see FIG. 6). A mid region 107 is located between thenear region 105 and the remote region 106. The measurement space 301 islocated within the observation region 302 of the camera 103. In FIG. 4,the limits of the observation region 302 are indicated by a dash-dottedline. The observation region 302 has not been plotted in the remainingfigures in order to simplify the representation.

Overall, it is possible to define three different coordinate systems,the base coordinate system 501, the camera coordinate system 502 and thecoordinate system of the calibration object 503. The relative positionand alignment of the measurement space 301 always remain unchangedwithin the camera coordinate system 502. To create the camera model, thecamera to calibration object 504 vector, that is to say the vectorbetween the coordinate origin of the camera coordinate system 502 andthe coordinate origin of the coordinate system of the calibration object503, is estimated with a pose estimate and an initial camera model to bedefined in any desired way, for example a nominal pinhole camera model,which specifies the pose of the calibration object 300 in relation tothe camera 103 for an initial pose P0. The camera to calibration object504 vector defines the initial pose P0 and the poses P1, P2, P3, . . . ,PN of the calibration object 300.

After the start of the method, the calibration object 300 is positionedwith an initial pose P0 in an observation region 302 of the camera 103of the surgical microscope 100 in method step S1 (see FIG. 4). Theinitial pose P0 is defined by the associated vector between thecoordinate origin of the camera coordinate system 502 and the coordinateorigin of the coordinate system of the calibration object 503. In theexemplary embodiment, the calibration object 300 arranged in the initialpose P0 is located within the measurement space 301. However, apositioning outside of the measurement space 301 would also be possiblefor as long as the calibration object 300 is located in the observationregion 302 of the camera 103.

Optionally, the calibration object 300 can be positioned freely inspace, for example placed on a table. In this case, the initial pose P0is estimated with an initial camera model.

In method step S2, the pose delta deltaP that is required to reach thefirst pose P1 in the measurement space 301 starting from the initialpose P0 is subsequently determined. In method step S3, the calibrationobject 300 is positioned in the first pose P1. To this end, the camera103 is moved in accordance with the pose delta deltaP such that the poseof the calibration object 300 changes accordingly.

Subsequently, a recording of the calibration object 300 in the firstpose P1 is made with the camera 103 in method step S4. On the basis ofthe recording, the characteristic markings of the calibration object 300can be identified and evaluated subsequently within the scope ofcreating the camera model.

Subsequently, the calibration object 300 is positioned in themeasurement space 301 in a further pose P2 (method step S5) and anotherrecording of the calibration object 300 is made with the camera 103(method step S6). Method steps S5 and S6 are repeated until thecalibration object 300 has been positioned in the desired number ofposes P1, P2, P3, . . . , PN and the corresponding recordings were made.

In subsequent method step S7, the camera model is created on the basisof the recordings made. To this end, important parameter values, forexample the distance between the image sensor of the camera 103 and theoptical center and the distortion coefficients, are determined. Inmethod step S7, the camera model or the parameters contained therein areused to estimate a pose of an object in the measurement space 301 of thecamera 103.

With reference to FIGS. 6 to 8, the selection of the poses P1, P2, P3, .. . , PN is explained in more detail below. According to an aspect ofthe disclosure, provision is made for the first pose P1 and the furtherposes P2, P3, . . . , PN to be chosen with such a distribution in themeasurement space 301 that a camera model is obtained which isrepresentative in relation to the entire measurement space 301. Thespecific sequence in which the poses P1, P2, P3, . . . , PN are adoptedis irrelevant to the camera model to be created since the recordings ofall poses P1, P2, P3, . . . , PN are taken into account with equalweighting in the camera model. However, a typical sequence of poses P1,P2, P3, . . . , PN may arise for reasons of saving time, that is to sayit may be advantageous to use the pose with the smallest distance fromthe current pose P1, P2, P3, . . . , PN as next pose P1, P2, P3, . . . ,PN. In principle, any pose P1, P2, P3, . . . , PN can be used as firstpose P1.

In the exemplary embodiment, this is achieved by virtue of thecalibration object 300 being positioned in a total of eight poses P1,P2, P3, P4, P5, P6, P7, P8 and corresponding recordings being made. Ofthese poses, four poses P1, P2, P3, P4 are in the near region 105 (seeFIGS. 7A to 7D) and four poses P5, P6, P7, P8 are in the remote region106 (see FIGS. 8A to 8D). The four poses P1, P2, P3, P4 in the nearregion 105 are arranged in a plane and the four poses P5, P6, P7, P8 inthe remote region are arranged in a plane. In twos, the poses P1, P2,P3, P4, P5, P6, P7, P8 are moreover arranged rotationally symmetricallyin relation to the optical axis OA (poses P1 and P4 and poses P2 and P3in FIG. 7, and poses P5 and P6 and poses P7 and P8 in FIG. 8). Anaveraged camera model, in which no region is overweighted, is created byvirtue of using the same number of recordings in the near region 105 andin the remote region 106 for the creation of the camera model.

Optionally, additionally poses P9, P10, P11, P12 may be recorded in themid region 107 (see FIG. 6) in order to be able to obtain a more robustcamera model. The overall number of the poses P1, P2, P3, P4, P5, P6,P7, P8 used in the exemplary embodiment should merely be understood tobe exemplary. Overall, more or fewer poses P1, P2, P3, . . . , PN can beused depending on the required quality of the camera model, for as longas these poses are chosen in such a way that a camera model that isrepresentative in relation to the entire measurement space is obtained.

Moreover, all poses P1, P2, P3, . . . , PN are chosen in such a way inthe exemplary embodiment that the calibration object 300 is fullypositioned within the measurement space 301 in each pose P1, P2, P3, . .. , PN. The various poses P1, P2, P3, . . . , PN are positioned in themeasurement space 301 by changing the position of the camera 103. Thismeans that the pose P1, P2, P3, . . . , PN of the calibration object 300remains unchanged in relation to the base coordinate system 501 whilethe position of the camera 103 is changed in order to change theposition of the measurement space 301 and consequently also the pose P1,P2, P3, . . . , PN of the calibration object 300 in the measurementspace 301. As described with reference to FIGS. 1 and 2, the position ofthe camera 103 is changed with the stand 201. This facilitatesautomation of the method for creating the camera model.

An exemplary method for verifying a camera model is described below withreference to FIGS. 9 to 11. By way of example, this can be the cameramodel obtained above with reference to FIGS. 3 to 8. For elucidation ofthe surgical microscope 100, reference is made to the explanations inrelation to FIGS. 1 and 2.

In a first method step S9, the current pose P_curr of the calibrationobject 300 positioned in the measurement space 301 of the camera 103 ofthe surgical microscope 100 is determined. Expressed differently, thecurrent relative pose of the calibration object 300 in relation to thecamera 103 is determined on the basis of an available camera model whichis intended to be verified (see FIG. 10). A desired target pose P_targetfor the calibration object 300 in the measurement space 301 is definedin the next method step S10. Method steps S9 and S10 can also be carriedout in reverse sequence or simultaneously.

Subsequently, the pose delta deltaP for reaching the target poseP_target starting from the current pose P_curr is determined in methodstep S11. Expressed differently, the vector according to which thecalibration object 300 needs to be moved to convert the current poseP_curr into the target pose P_target is determined.

In method step S12, the current pose P_curr of the calibration object300 is changed in accordance with the determined pose delta deltaP.Optionally, this can be carried out manually or in automated fashion. Inthe case of a manual execution, the determined pose delta deltaP can bedisplayed on a monitor. Subsequently, the camera 103 is manuallydisplaced by a user, for example a service technician, a hospital staffmember, an assembly colleague, in such a way that the calibration object300 is positioned in the measurement space 301 in accordance with thetarget pose P_target. In the case of an automated embodiment, the posedelta deltaP is controlled with the robotic stand 201, which moves thecamera 103 accordingly. FIG. 11 shows the calibration object 300 in theactually reached target pose P_target_act and the movement path (dashedarrow) of the camera 103 for reaching the actual target poseP_target_act.

Subsequently, the deviation between the defined target pose P_target andthe actual target pose P_target_act is determined in method step S13 andcompared to a limit for a maximally admissible deviation (method stepS14). Expressed differently, a check is carried out as to whether theactual target pose P_target_act has the desired value, that is to saywhether the difference between the target pose P_target and the actualtarget pose P_target_act is below a set limit or within a specifiedtolerance range. The determined deviation can be considered a measurefor the quality of the camera model.

Method steps S9 to S14 can subsequently be repeated for a specifiablenumber of target poses P_target in order to be able to obtain morereliable statements about the current calibration. To this end, targetposes P_target can be chosen in the near region 105 and in the remoteregion 106, and optionally additionally in the mid region 107. Thesequence of the target poses P_target can be chosen freely. The sequenceof the target poses P_target can typically be chosen in such a way thatthe duration of a calibration journey, i.e., the time taken to home inon all desired target poses P_target, is as short as possible.

If the limit is exceeded, that is to say the demanded quality is notachieved, the camera model can be created anew, for example using themethod explained above with reference to FIGS. 3 to 8.

FIG. 12 shows an exemplary arrangement 400 in a schematicrepresentation. This arrangement 400 can be used to carry out themethods described above with reference to FIGS. 3 to 11 and can bedesigned accordingly.

The arrangement 400 includes a surgical microscope 100 with a camera 103and a stand 201. For more details, reference is made in exemplaryfashion to FIGS. 1 and 2 and the associated description. Moreover, thearrangement includes means 410 which are suitable for carrying out thesteps of a method for creating a camera model for a camera of a surgicalmicroscope 100, of a method for estimating a pose of an object in ameasurement space 301 of the camera 103 of the surgical microscope 100and of a method for verifying the camera model. These means 410 includea control unit 401, a processing unit 405, and a memory unit 406, whichare operatively signal connected to one another, indicated in FIG. 11 bydouble-headed arrows.

As already explained with reference to FIGS. 1 and 2, the control unit401 can output control signals 403, 404 to the camera 103 and the stand201. This firstly facilitates the triggering of the camera 103 requiredto make the recordings of the calibration object 300 and secondlyfacilitates the positioning of the camera 103 by an appropriate movementof the stand 201. Furthermore, there is an operative signal-connectionbetween the camera 103 and the means 410, for example in order to beable to store recordings of the camera 103 in the memory unit 406 andprocess these in the processing unit 405.

The signal transmission can be implemented in wired or wireless fashionin each case, for example using radio signals. Correspondingtransmission and reception devices are not shown in FIG. 12, but theymay have a design that is conventional in the art. As a consequence, themeans 410 need not necessarily be arranged spatially adjacent to thesurgical microscope, but may also be present remotely, e.g., incentralized fashion. This also facilitates a common use of the means 410by a plurality of surgical microscopes 100. Additionally, the means 410need not necessarily be arranged in spatially adjacent fashion. By wayof example, there is the option of the control unit 401 being arrangedspatially adjacent to the surgical microscope 100 (see FIG. 1) while thememory unit 406 and the processing unit 405 may be arranged spatiallyremotely.

FIG. 13 shows a surgical microscope 100 which has a further camera 108in addition to the camera 103, which is a microscope camera. The furthercamera 108 is a surround camera with an associated coordinate system505. In FIG. 13, the observation region 303 of the further camera 108 isdepicted by a dash-dotted line. The camera 103 and the further camera108 are securely connected to one another by way of a connection 109such that the pose of the two cameras 103, 108 with respect to oneanother is unchangeable and a rigid transformation between the twocameras 103, 108 is facilitated.

As described above, a camera model can be created for the camera 103.Optionally, a camera model can additionally be created for the furthercamera 108 by virtue of implementing a suitable geometric transformationfrom the coordinate system 502 of the camera 103 to the coordinatesystem 505 of the further camera 108.

Overall, the present disclosure offers, inter alia, the followingadvantages:

The process for creating the camera model is fully automatable. As aresult, a higher reproducibility can be obtained vis-à-vis randomrecordings.

Recordings of the calibration object in the measurement space chosen intargeted fashion facilitate the creation of a camera model that isrepresentative for the entire measurement space.

The accuracy of the camera model can be increased since poses of thecalibration object (vector from camera to calibration object) can behomed in on in targeted fashion.

The provided method for creating the camera model only requires onecalibration object with a known geometry. The calibration object mayalso be depicted on a monitor.

Should the camera model be changed as a result of the influence oftemperature or transport, there can be an automated verification of thecamera model in the field, for example by the hospital staff or withinthe scope of remote maintenance. The number of poses could be reduced tothis end. This reduces the amount of time required.

In the case of automation, there is no need for manual movement of thecalibration object or surgical microscope.

The present disclosure has been explained in detail on the basis ofexemplary embodiments for explanatory purposes. However, a personskilled in the art will appreciate that they may depart from details ofthese exemplary embodiments.

Since it is possible to deviate from the individual described exemplaryembodiments in a manner evident to a person skilled in the art, thepresent disclosure should not be restricted by the described exemplaryembodiments, but merely by the attached claims.

The expression “and/or” used here, when it is used in a series of two ormore elements, means that any of the elements listed can be used alone,or any combination of two or more of the elements listed can be used.

LIST OF REFERENCE NUMERALS

-   100 Surgical microscope-   102 Optical observation unit-   103 Camera-   104 Eyepiece-   105 Near region-   106 Remote region-   107 Mid region-   108 Further camera-   109 Connection-   201 Stand-   202 Actuating device-   205 Stand base-   206 Rollers-   207 Foot brake-   208 Stand column-   209 Support arm-   210 Spring arm-   211 Mount for the optical observation unit-   213 Connection element-   214 Holding arm-   215 Swivel arm-   300 Calibration object-   301 Measurement space-   302 Observation region of the camera-   303 Observation region of the further camera-   400 Arrangement-   401 Control unit-   403 Control signal-   404 Control signal-   405 Processing unit-   406 Memory unit-   410 Means-   501 Base coordinate system-   502 Camera coordinate system-   503 Coordinate system of the calibration object-   504 Vector from camera to calibration object-   505 Coordinate system of the further camera-   A Axis of rotation-   B Axis of rotation-   C Tilt axis-   D Axis of rotation-   E Swivel axis-   F Tilt axis-   OA Optical axis-   P0 Initial pose-   P1, P2, . . . , PN Pose-   P_curr Current pose-   P_target Defined target pose-   P_target_act Actual target pose-   deltaP Pose delta-   S1 to S14 Method steps

What is claimed is:
 1. A method for creating a camera model for a cameraof a surgical microscope, the method comprising: positioning acalibration object in an initial pose in an observation region of thecamera of the surgical microscope; determining a pose delta for reachinga first pose for the calibration object in a measurement space of thecamera starting from the initial pose, the measurement space beingdefined as a volume to be observed with the camera; positioning thecalibration object in the first pose in accordance with the pose delta;making a first recording of the calibration object in the first posewith the camera; positioning the calibration object in at least onefurther pose in the measurement space of the camera; making a secondrecording of the calibration object in the at least one further posewith the camera; creating the camera model based on the first and secondrecordings; and the first pose and the at least one further pose beingchosen with a distribution in the measurement space such that a cameramodel is obtained which is representative in relation to the entiremeasurement space.
 2. The method according to claim 1, wherein a firstnumber of poses are located in a first region near to the camera and asecond number of poses are located in a second region remote from thecamera.
 3. The method according to claim 2, wherein the first number ofposes in the first region near to the camera corresponds to the secondnumber of poses in the second region remote from the camera.
 4. Themethod according to claim 1, wherein the initial, first, and at leastone further poses are selected such that the calibration object ispositioned fully within the measurement space in each of the initial,first, and at least one further poses.
 5. The method according to claim1, wherein characteristic markings of the calibration object locatedoutside of the measurement space remain unconsidered when creating thecamera model.
 6. The method according to claim 1, wherein thecalibration object is positioned in the measurement space by changingthe position of the camera.
 7. The method according to claim 1, whereinthe measurement space has a cylindrical or cuboid form.
 8. The methodaccording to claim 1, wherein: the camera model is a first camera model,and the method further comprises creating a further camera of thesurgical microscope based on the first camera model.
 9. The methodaccording to claim 1, wherein the calibration object is positionedfreely in space.
 10. A method for estimating the pose of an object inthe measurement space of the camera of a surgical microscope, the methodcomprising: estimating the pose with the camera model for the camera ofthe surgical microscope, the camera model having been created with themethod according to claim
 1. 11. A method for verifying the cameramodel, the camera model having been created with the method according toclaim 1, the method comprising: determining a current pose of thecalibration object positioned in the measurement space of the camera ofthe surgical microscope; defining a target pose for the calibrationobject; determining the pose delta for reaching the target pose startingfrom the current pose; changing the current pose of the calibrationobject in accordance with the pose delta; determining a deviationbetween the target pose and an actual target pose; and comparing thedeviation with a limit for a maximum admissible deviation.
 12. Anarrangement, comprising: a surgical microscope having a camera, andmeans for carrying out the method according to claim
 1. 13. Thearrangement according to claim 12, wherein the camera is a surroundcamera or a microscope camera.
 14. A non-transitory computer-readablestorage medium encoded with a computer program comprising executablecommands that when executed by the arrangement according to claim 12cause the arrangement to: position a calibration object in an initialpose in an observation region of the camera of the surgical microscope;determine a pose delta for reaching a first pose for the calibrationobject in a measurement space of the camera starting from the initialpose, the measurement space being defined as a volume to be observedwith the camera; position the calibration object in the first pose inaccordance with the pose delta; make a first recording of thecalibration object in the first pose with the camera; position thecalibration object in at least one further pose in the measurement spaceof the camera; make a second recording of the calibration object in theat least one further pose with the camera; create a camera model basedon the first and second recordings; and the first pose and the at leastone further pose being chosen with a distribution in the measurementspace such that a camera model is obtained which is representative inrelation to the entire measurement space.